The minute dosage required to produce the psychological effect of LSD suggested that the mode of action was due to an anti-enzyme action. Since the only substrate metabolized by the brain cell is glucose, an investigation was made of the effect of LSD on carbohydrate metabolism in vivo. The increase in hexosemonophosphate and glucose suggest there is an increased metabolism of glycogen coupled with a probable block in the breakdown of hexo-semonophosphate. The changes in carbohydrate metabolism observed after the intake of LSD are considered responsible for at least part of the psychological symptoms.